Circuitry is universally used in many fields. It is applicable to fluidics, optics, electronics, etc.
In the field of electronics, circuitry is used to define, control, and direct operations, processing, sensing, etc. Often the topology of a circuit will define its function.
For example, an embedded controller may be built into a product and capable of providing control to devices hooked up external to the controller. Interfaces with such external devices may be classified depending on direction of control/sensing as inputs, outputs, or bidirectional. The particular configuration of inputs and outputs in embedded controllers may either be fixed or customizable. For example, in a washing machine, the embedded controller might have inputs from the temperature buttons or dials and outputs to control various motors and valves within the machine. In this way, the embedded controller provides a method for its embedded computer to interact with the outside or real world.
Embedded controllers with a fixed configuration of inputs and outputs are generally purchased off the shelf and are quickly available from manufacturers' stock. Such controllers often utilize a modular system of input/output interfaces to provide a method to customize the number and type of input output interfaces. For example, an expansion bus may be used to which the modular input/output (I/O) devices connect. This extra overhead adds to the cost of the controller. This presents a problem.
An embedded controller with just the correct number of needed I/O ports requires a custom configuration that must be specially designed and manufactured. This requires considerable up front engineering costs. Thus, custom configuration embedded controllers are generally not an option unless production volume is high enough to amortize the initial investment. A production volume of 500 units per year is often suggested as the volume needed to justify this initial cost. This presents a problem.